Preview button added by node module in NodeForm as
action in actions method:
$element['preview'] = [
'#type' => 'submit',
'#access' => $preview_mode != DRUPAL_DISABLED && ($node->access('create') || $node->access('update')),
'#value' => t('Preview'),
'#weight' => 20,
'#submit' => ['::submitForm', '::preview'],
Taxonomy entity form created by TermForm.php so i think you should:
TermForm.php in another Class.
- Create a
preview method like the node preview method but for the term.
- Add a route subscriber to tell drupal to load your form not the default.